Fix isMutatingPosts example

Previously the example was using an array as argument, which doesn't match the newest API.

Author: sakarisson

docs/src/pages/reference/useIsMutating.md

@@ -10,7 +10,7 @@ import { useIsMutating } from 'react-query'
 // How many mutations are fetching?
 const isMutating = useIsMutating()
 // How many mutations matching the posts prefix are fetching?
-const isMutatingPosts = useIsMutating(['posts'])
+const isMutatingPosts = useIsMutating({ mutationKey: 'posts' })
 ```
 
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